1. Field of the Invention
The present invention relates to a fuel cell system using hydrogen as a fuel, and specifically to a fuel cell system having a characteristic in a residual hydrogen treatment when a system is at a halt.
2. Description of the Related Art
Conventionally, in a fuel cell system for a vehicle in which hydrogen is employed as its fuel, as a treatment technique for residual hydrogen of the time of a system halt, one described in, for example, Japanese Patent Application Laid-Open No. 11-26003 is known. In this conventional fuel cell system, residual hydrogen is consumed first by a discharge resistor during the system halt so as to decrease the voltage of a fuel cell stack, and when the voltage becomes a certain output voltage or below, purge is performed by means of an inert gas such as nitrogen so as to make the output voltage 0 volts.
However, in this conventional fuel cell system""s case, since the fuel cell stack is purged by an inert gas for the residual hydrogen treatment, the inert gas has to be carried in the system, and it is not preferable to carry an inert gas which does not directly contribute to a fuel cell electrical generation in terms of weight, cost, layout, refilling, and the like, particularly when it is applied to a system in which a miniaturization and weight saving are necessary such as in a fuel cell system for a vehicle.
The present invention has been achieved by considering such conventional problems, and it is an object of the present invention to provide a fuel cell system in which residual hydrogen can be consumed until the output voltage becomes 0 volts and miniaturization and weight saving of the system can be achieved without carrying a substance such as an inert gas which does not directly relate to a fuel cell electrical generation.
According to the present invention, the fuel cell stack is connected with a discharging resistor to consume residual hydrogen at the time of a system halt, and the discharging resistor is switched to restrict a discharging current when the output voltage becomes a fixed value or below to consume residual hydrogen until the output voltage becomes 0 volts.
A first aspect of the present invention provides a fuel cell system in which fuel supply to a fuel cell is cut at the time of the system halt and the fuel cell output is connected with a discharging resistor so as to consume residual hydrogen in a fuel cell stack, wherein the system comprises a plurality of discharging resistors and a switching mechanism selecting and connecting a discharging resistor capable of consuming high power at the time of the system halt and selecting and connecting a discharging resistor in which merely a minute current flows when the fuel cell output becomes a fixed value or below.
Thus, since the discharging resistor capable of consuming high power is selected and connected immediately after the system is halted and the discharging resistor in which merely a minute current flows is selected and connected when the fuel cell output becomes a fixed value or below, discharging can be performed until the output voltage becomes 0 volts without causing damage to the fuel cell, and residual hydrogen can be consumed.
According to a second aspect of the present invention, the discharging resistors are provided in a humidification water tank for humidifying hydrogen supplied to the fuel cell.
Accordingly, in addition to the first aspect, since discharging of heat energy by means of the discharging resistors is performed in water inside the water tank, the discharging resistors do not become red-hot by conducting electricity, and a bad influence on fuel hydrogen can be prevented.
According to a third aspect of the present invention, a state where the discharging resistor is connected with the fuel cell output is maintained even after the system is halted. Thus, even when hydrogen is leaked to the fuel cell during the time of the system halt and a small amount of fuel cell electricity is generated, hydrogen can be consumed by the discharging resistor.